54 ANNALS NEW YORK ACADEMY OF SCIENCES 



oped as far back in geologic time as the Triassic, since we have in Jurassic 

 time a shell with rounded volutions and an ornament of three simple 

 spirals, which was probably developed from a two-spiraled form just as 

 the mutation trispiralc. described below, arose from the type species by 

 the development of another spiral on the shoulder. 



Cerithium bicarinatum mut. trispirale n. mut. 

 Plate II, fig. 7. 



1824. Ccrithiitiii hirariiiatiini Deshayes. Dese. des eoqiiilles foss. des environs 



de Paris, p. 35G, pi. 53, fig. 6. 

 18GG. Cerithitini hicarinatiwi Deshayes, Desc. des aniin. siins vert, decouverts 



dans le bassin de Paris, p. 180. 

 Measurements : Length, 25 mm. ; greatest diameter, 10.2 mm. ; apical angle, 

 25° ; sutural angle, 85°. 



The development of the variety is precisely the same as that of the 

 type species to the seventh volution, on which a faint third spiral is in- 

 troduced above the two primary ones. This spiral grows rapidly until 

 it equals in strength the lower primary spiral. The median spiral is 

 slightly stronger than the others, so that the outline of the volution 

 would be a regular curve if represented by a line touching the edges of 

 the spirals. This type of ornamentation persists throughout the re- 

 mainder of shell growth. The body volution has two strong spirals and 

 three or more fine ones below the three primary spirals. The aperture is 

 like that of the type species. 



Horizon and localities: Sables Moyens (Tipper Eoeenic). Acy-en-Multien 

 and other localities in the Paris Basin. 

 No. 20145, Columbia University colleotion. 



Remakks : This mutation differs from the type species only in the 

 presence of a third spiral, which becomes strong enough to change the 

 outline of the volution. There is a perfect gradation between the species 

 and its mutation, since specimens of the former are found in which the 

 third spiral is barely distinguishable as a faint elevation on the adult 

 whorls, and others in which this feature appears earlier and earlier in 

 the ontogeny until the typical form of the mutation is developed. The 

 mutation represents merely the next step in the evolution of this group. 



Cerithium retardatum sp. nov. 



Plate II, fig. 6. 



Measurements: Length. 27.2 nmi. ; greatest diameter, 1.5 mm.; apical angle, 

 2i° ; sutural angle, 80°. 



